Optical disk and optical disk apparatus

ABSTRACT

An optical disk with sectional trapezoidal pits comprising a substrate having information recorded by a plurality of pit trains formed thereon at a specified track pitch, and a reflective layer formed on the substrate, wherein the information is reproduced by being irradiated with light beam via an objective lens, the track pitch is set within the range of (0.72 to 0.8) α×λ/NA/1.14 μm when a wavelength of the light beam is μ nm and a numerical aperture of the objective lens is NA, each of the pits is a multiplication ratio used to secure allowable disk tilt angles in an upper width within the range of (0.3 to 0.50) α×λ/NA/1.14 μm, a bottom width within the range of (0.2 to 0.32) α×λ/NA/1.14 μm and a depth within the range of (1/4.2×λ/n) to (1/5.2 λ/n) (n: refractive index of said substrate and λ: a wavelength) and obtained by 2.623×10 -7  ×(d/λ) 2  -1.706×10 -4  (d s  /λ)+0.934, where d s  is thickness of the substrate.

CROSS-REFERENCE TO THE RELATED APPLICATION

This is a continuation of application Ser. No. 08/778,313, filed Jan. 2,1997 now U.S. Pat. No. 5,777,981 which is a continuation of applicationSer. No. 08/541,598, filed Oct. 10, 1995, now U.S. Pat. No. 5,602,825;which is a continuation-in-part of application Ser. No. 08/475,494,filed Jun. 7, 1995, now U.S. Pat. No. 5,592,464; which is a continuationof application Ser. No. 08/304,849, filed Sep. 13, 1994, now U.S. Pat.No. 5,459,712.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical disk on which information isrecorded in pits with high density and an optical disk apparatuscontaining the optical disk and a playback optical system.

2. Description of the Related Art

With the recent advances in image digital signal processing techniquesand moving-picture compression techniques, the latter of which have beendeveloped by such a standardizing organization as the MPEG (MovingPicture Image Coding Experts Group), there is a growing expectation ofthe advent of an optical disk capable of reproducing moving-pictureinformation such as a movie for two hours and being the same size as aCD (compact disk) in place of a VTR or laser disk. The recordingcapacity required to record two hours of moving-picture information inthe form of analog video signals by a standard TV system such as NTSC ason the laser disk, amounts to 80 Gbyte including sound. Use ofmoving-picture compression techniques prescribed by a standardizedmethod called MPEG-2, for example, requires as small a capacity asnearly 4 Gbyte even for a picture quality as good as a highpicture-quality VTR such as S-VHS. The 4-Gbyte disk has been put intopractical use in the form of a 300-mm diameter write-once read-manyoptical disk. As more and more optical disks will be used in homes inthe future, it is needed to achieve an easy-to-use 120-mm diameter diskwhich has the same size and almost the same capacity as the CD.

The capacity of the CD format presently available as the music CD or theCD-ROM is 790 Mbyte at the maximum (when the linear velocity is 1.2m/s). The capacity of this order can store only 24 minutes of compressedmoving-picture information by MPEG-2. Thus, to store two hours ofcompressed moving-picture information by MPEG-2 with the CD size, therecording density must be made five times as high as that of the CD. Inthe current CD format, the substrate thickness is 1.2 mm, the trackpitch is 1.6 μm, the pit pitch is 1.66 μm when the linear velocity(relative velocity between light beam and disk=disk's circumferentialvelocity) is 1.2 m/s, the bit length is 0.59 μm, and the modulationmethod is EFM (eight to fourteen modulation). In the playback opticalsystem, the playback semiconductor laser, or the laser diode (LD) has awavelength of 780 nm, the object lens has an NA (numerical aperture) of0.45, and the beam spot has a diameter of 1.4 μm. The beam spot diameteris selected mainly from the standpoint of avoiding the effect ofcrosstalk between adjacent tracks.

To increase the recording density of the optical disk requirestechniques for forming small pits in the disk and those for making thebeam spot size small on the optical disk in the playback optical system.Concerning techniques for forming pits, for example, an optical diskmatrix recording technique using Kr ion laser light (ultraviolet rays)with a wavelength of 351 nm has been proposed (The 1993 Autumn NationalConvention of the Applied Physics Society, 28-SF-2). This techniquemakes it possible to form smaller pits than a conventional Ar ion laser.In the playback optical system, by making the wavelength of the playbacklaser beam shorter and increasing the NA, the beam spot diameter can bemade smaller. Actually, however, with conventional techniques used in CDplayers, even if a short wavelength light source such as a red laserdiode were used, the capacity would be increased by 1.5 times at most.With such an increase in the capacity, it cannot be expected to increasethe capacity by five times that of an ordinary CD, which is what isrequired to record two hours of compressed moving-picture information.

As described above, with the conventional optical disk techniques, toavoid the problem of crosstalk between adjacent tracks, the track pitchand pit pitch are set larger than the beam spot diameter of the playbacklight beam. As a result, only by making the wavelength of playback lightbeam shorter and increasing the NA of the object lens, the recordingdensity cannot be raised to the extent that the capacity required tostore two hours of compressed moving-picture information by MPEG2 withthe CD size, for example.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an optical disk and anoptical disk apparatus which can lessen crosstalk between adjacenttracks to the extent that there is no problem in practical use, even ifthe track pitch and pit pitch are smaller than the beam spot diameter ofthe playback light beam, and which achieves a higher density and agreater capacity than in the prior art.

According to the present invention, there is provided an optical diskcomprising a substrate and a recording layer which is formed on thesubstrate and on which information is recorded at specific pitches inthe form of pit trains, wherein the information is reproduced byprojecting a light beam via an object lens, and when the wavelength ofthe light beam is λ μm and the numerical aperture of the objective lensis NA, the track pitch is set in the range of (0.72 to 0.8)×λ/NA/1.14μm, and each of the pits has a trapezoidal cross section whose upperwidth is in the range of (0.3 to 0.5)×λ/NA/1.14 μm and whose lower widthis in the range of (0.2 to 0.32)×λ/NA/1.14 μm.

According to the present invention, there is provided an optical diskapparatus comprising an optical disk comprising a substrate and arecording layer which is formed on the substrate and on whichinformation is recorded at specific pitches in the form of pit trains,an objective lens provided so as to face the optical disk, means forprojecting a light beam onto the optical disk via the objective lens,and means for sensing the reflected light of the light beam projected onthe optical disk by the projecting means to reproduce the informationrecorded on the optical disk, wherein when the wavelength of the lightbeam is λ μm and the numerical aperture of the objective lens is NA, thetrack pitch is set in the range of (0.72 to 0.8)×λ/NA/1.14 μm, and eachof the pits has a trapezoidal cross section whose upper width is in therange of (0.3 to 0.5)×λ/NA/1.14 μm and whose lower width is in the rangeof (0.2 to 0.32)×λ/NA/1.14 μm.

According to the present invention, there is provided an optical diskcomprising a substrate and a recording layer which is formed on thesubstrate and on which information is recorded at specific pitches inthe form of pit trains, wherein the information is reproduced byprojecting a light beam via an objective lens, and when the wavelengthof the light beam is λ μm and the numerical aperture of the object lensis NA, the track pitch is set in the range of (0.72 to 0.8)×λ/NA/1.14μm, and each of the pits has a trapezoidal cross section whose upperwidth is in the range of (0.3 to 0.5)×λ/NA/1.14 μm and whose inner wallhas an angle of 30° to 60°.

According to the present invention, there is provided an optical diskapparatus comprising an optical disk comprising a substrate and arecording layer which is formed on the substrate and on whichinformation is recorded at specific pitches in the form of pit trains,an objective lens provided so as to face the optical disk, means forprojecting a light beam onto the optical disk via the object lens, andmeans for sensing the reflected light of the light beam projected on theoptical disk by the projecting means to reproduce the informationrecorded on the optical disk, wherein when the wavelength of the lightbeam is λ μm and the numerical aperture of the objective lens is NA, thetrack pitch is set in the range of (0.72 to 0.8)×λ/NA/1.14 μm, and eachof the pits has a trapezoidal cross section whose upper width is in therange of (0.3 to 0.5)×λ/NA/1.14 μm and whose inner wall has an angle of30° to 60°.

Furthermore, the invention provides an optical disk having sectionaltrapezoidal pits comprising a substrate having information recorded by aplurality of pit trains formed thereon at a specified track pitch, and areflective layer formed on the substrate, wherein the information isreproduced by being irradiated with light beam via an objective lens,the track pitch is set within the range of (0.72 to 0.8) α×λ/NA/1.14 μmwhen a wavelength of the light beam is λ nm and a numerical aperture ofthe objective lens is NA, each of the pits is magnified by amultiplication ratio α used to secure allowable disk tilt angles in anupper width within the range of (0.3 to 0.50) α×λ/NA/1.14 μm, a bottomwidth within the range of (0.2 to 0.32)α×λ/NA/1.14 μm and a depth withinthe range of (1/4.2×λ/n) to (1/5.2×λ/n) (n: refractive index of saidsubstrate and λ: a wavelength 0.65 μm) and α obtained by 2.623×10⁻⁷×(d_(s) /λ)² -1.706×10⁻⁴ (d_(s) /λ)+0.934m (d_(s) : thickness of thesubstrate).

By setting various parameters of the pit shape at the above-describedvalues, the amount of crosstalk between adjacent tracks is suppressed toless than -20 dB, which must be met to restore the original informationfrom the reproduced signal, and the playback signal level and the levelof the push-pull signal for tracking are maintained sufficiently.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic view to help explain the shape of a pit in anoptical disk according to an embodiment of the present invention;

FIG. 2 is a schematic diagram of the playback optical system in anoptical disk apparatus;

FIG. 3 is a diagrammatic view of the pit arrangement on the optical diskfor calculating the levels of the playback and push-pull signals sensedfrom the optical disk;

FIG. 4 shows the relationship between the playback signal level and thepush-pull signal level obtained from calculations using the pit size inthe track width direction and the pit depth as parameters;

FIG. 5 is a diagrammatic view of the pit arrangement on the optical diskused to evaluate the MTF of the playback optical system and crosstalkbetween adjacent tracks;

FIG. 6 shows the dependence of the playback optical system's MTF and thecharacteristics of crosstalk between adjacent tracks on the pit shapewhen the upper width Wm and lower width Wi of the pit are changedvariously;.

FIG. 7 shows the dependence of the playback optical system's MTF and thecharacteristics of crosstalk between adjacent tracks on the pit shapewhen the conditions are the same as in FIG. 6 except that λ is set at0.650 μm;

FIG. 8 shows the dependence of the playback optical system's MTF and thecharacteristics of crosstalk between adjacent tracks on the pit shapewhen the upper width Wm and the angle θ of the pit's inner wall arechanged variously;

FIG. 9 shows the dependence of the playback optical system's MTF and thecharacteristics of crosstalk between adjacent tracks on the pit shapewhen the conditions are the same as in FIG. 8 except that the upperwidth Wm of pit is fixed at 0.35 μm;

FIG. 10 shows the dependence of the playback optical system's MTF andthe characteristics of crosstalk between adjacent tracks on the tilt;

FIG. 11 shows the dependence of the NA of the object lens on the tiltwhen an optical disk whose substrate thickness is 1.2 mm is used;

FIG. 12 shows the dependence of the NA of the object lens on the tiltwhen an optical disk whose substrate thickness is 0.6 mm is used;

FIG. 13 shows a cross sectional outline of an actual pit;

FIG. 14 shows a relationship between the groove width of an actual pitand the groove width of a model pit;

FIG. 15 shows a relationship between an angle of inclination of anactual pit and that of a model pit;

FIGS. 16A and 16B are a perspective view and a sectional view of anoptical disk according to an embodiment of the present invention; and

FIG. 17 is a block diagram of an optical disk apparatus according to anembodiment of the present invention;

FIG. 18 is a graph showing a relationship between inclination of a diskand aberration;

FIG. 19 is a graph showing a relationship between a disk tilt angle anda window occupation ratio; and

FIG. 20 is a graph showing a relationship among an allowable disk tiltangle, a ratio of (a substrate thickness/a wavelength) andmultiplication ratios.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before explanation of an embodiment, the basic concept of the presentinvention will be described.

To make the density of the optical disk higher, a spot diameter of theplayback light beam must be made smaller. To do this, it is essential tomake the wavelength of the playback laser diode shorter and increase theNA of the objective lens. Laser diodes (self-pulsation-type laserdiodes) of a low-noise type whose wavelength is 0.685 μm and whoseoutput is several milliwatts have already been put into practical use.Laser diodes whose wavelength is 0.650 μm are getting close to practicaluse.

The NA of the objective lens is limited by ease of making a lens and thetilt angle between the lens and the disk. The smaller the lens load (thethinner the optical disk substrate, the smaller the lens load) and thesmaller the NA, the easier it is to make the objective lens. For anobjective lens whose NA is nearly 0.6, it is possible to make the beamspot diameter smaller even with a single nonspherical lens. However,with an objective lens used in the playback optical system for theoptical disk, coma aberration occurs due to the tilt between the opticaldisk and the playback light beam caused by the tilt of the optical diskor the tilt of the optical axis of the objective lens.

Specifically, an attempt to make the NA of the object lens larger inorder to make the spot size of the playback light beam smaller permitsthe aberration of the objective lens to increase sharply due to the tiltbetween the optical disk and the playback light beam. As the aberrationof the objective lens becomes larger, the amount of crosstalk betweenadjacent tracks increases accordingly, and the playback resolving powerdecreases. The thinner the optical disk substrate, the smaller theeffect of the tilt. In Japanese Journal of Applied Physics, Vol. 32(1993), pp. 5402-5405, changes in the shape of the spot of the playbacklight beam corresponding to the tilt when the substrate thickness is 1.2mm, the same as the CD, and 0.6 mm, with a wavelength of 0.690 μm andNA=0.6 are explained. According to this description, when the substratethickness is 1.2 mm, a tilt of 5 mrad lowers the center strength of thebeam spot as much as 10% and causes a rise in the side lobe andaberration, which contributes to crosstalk. In contrast, when thesubstrate thickness is 0.6 mm, the substrate can withstand the tiltsranging up to 10 mrad.

FIGS. 11 and 12 show the results of calculating the tilt characteristicswhen the substrate thickness (t) is 1.2 mm and 0.6 mm using the NA as aparameter. The abscissa represents the angle of tilt and the ordinateindicates the normalized peak intensity of the playback signal. Thewavelength (λ) of the playback light beam is assumed to be 0.690 μm.With the substrate thickness being 0.6 mm and the NA=0.6, the tilt is9.5 mrad and the peak intensity of the playback signal decreases by 10%.When the substrate thickness is 1.2 mm, the NA=0.49. Specifically, bychanging the substrate thickness from 1.2 mm (the conventional CD'sthickness) to 0.6 mm, the NA can be increased from 0.49 to 0.6, andconsequently the surface recording density can be increased by 1.5times. Because the spot size is proportional to λ/NA and the surfacerecording density is inversely proportional to the square of the spotsize, this gives (0.6/0.49)², which means that the area recordingdensity is 1.5 times as high as that of the conventional CD.

However, just making the substrate thickness thinner can cause thesubstrate to warp substantially due to temperature or humidity. The warpof the substrate contributes mainly to the tilt. To avoid this, it ismost effective to make the optical disk double-sided as the laser disk,or to give the optical disk a symmetrical structure with respect to thefront and back. In that case, it is possible to record information onboth sides. With a single-structure optical disk like the conventionalCD, since an aluminum reflective film or protective film is formed onone side of the substrate, the substrate has an asymmetrical moistureabsorption with respect to the front and back, and thus tends to warpeasily. A double-sided optical disk cancels the distortion of thesubstrate due to moisture absorption, thereby preventing a large tiltfrom occurring.

The evaluation results described above show that if a combination of alaser diode with a wavelength of 0.685 μm, a 0.6 mm thick substrate, andan objective lens with an NA=0.6 are used, the wavelength will beshortened from 0.780 μm to 0.685 μm and the NA will grow larger from0.45 to 0.6, so that the recording density can be made about 2.3 timesas large as that of the conventional CD format even by conventional CDdesign techniques. Specifically, because the spot size is proportionalto λ/NA, this gives (0.685/0.6)/(0.780/0.45), meaning that the recordingdensity is about 2.3 times as high as that of the conventional CD.However, to achieve the capacity required to record two hours ofcompressed moving-picture information by MPEG2 with the CD size, it isnecessary to make the recording density (capacity) about five times aslarge as that of the conventional CD format.

According to the present invention, there is provided an optical diskwhich enables the track pitch to be made much smaller in order toachieve a much higher density and greater capacity, while assuring thelow crosstalk characteristics and the sufficient signals levels of theplayback signal and the push-pull signal by optimizing the pit shape ofthe same beam spot size as described above. Hereinafter, the pit shapein the present invention will be described in detail.

FIG. 1 is an explanatory diagram of the shape of a pit in an opticaldisk according to the present invention. As shown in the figure, theshape of a pit 10 is approximated by a shape with a trapezoidal crosssection. The inner wall 11 of the pit 10 is inclined downward and itsbottom portion 12 is almost flat. Numeral 13 indicates the cross sectionof the pit 10 along the radius of the optical disk (the track widthdirection); 14 the cross section along the circumference of the opticaldisk (the track direction); Wm the size of the top of pit 10 across thetrack width (hereinafter, the upper width); Wi the size of the bottom ofthe pit 10 across the track width (hereinafter, the lower width); hm thedepth of the pit 10, Zm the length of the pit 10 along the track; and θrepresents the angle of the inner wall of the pit 10 (the angle that theinner wall forms with respect to the surface of the optical disk).

FIG. 2 shows a model of the playback optical system of the optical diskapparatus used in the analysis. The figure shows the incident lightdistribution 20 (V1 (x,y)) of playback light beam, incident light 21, apolarization beam splitter (or half mirror) 22 for separating theincident light 21 from the reflected light 26, an objective lens 23 witha numerical aperture of NA, the distribution 24 (V2 (x,y)) of focusedlight (beam spot) by the object lens on the recording surface (pitsurface) of the optical disk, an optical disk 25 with a complexreflectivity of r2 (x,y), the reflected light 26, and the distribution27 (V3 (x,y)) of the reflected light 26 on a photosensor.

FIG. 3 diagrammatically shows the pit arrangement on the optical diskused to calculate the levels of the playback signal and the push-pullsignal, where the track pitch (the pitch of a pit across the trackwidth) is Pt and the pit pitch (the pit pitch along the track) is Pmy.The beam spots 30 and 31 of the playback light beam represent spot A atthe center of a pit and spot B between pits, respectively. The amplitudeof the playback signal is represented as |S(A) - S(B)|, where S(A) andS(B) indicate the output signals of the photosensor when the beam spotis positioned at (A) and (B), respectively. Lines 32 and 33 show thepositions in which the push-pull signal (the difference signal betweenthe output signals of the split photosensors arranged along the track inat least two sensing areas) is obtained in an area (C) with pits and anarea (D) without pits. Those push-pull signals each have the averagep--p values in area C and area D.

FIG. 4 shows the results of calculating the levels of the playbacksignal and the push-pull signal using the size of a pit across the trackwidth and the pit depth as parameters with a playback laser beamwavelength of 0.685 μm, NA=0.6, Zm=0.5 μm, Pmy=1 μm, and Pt=0.72 μm. Thebeam filling factors A/W(X) and A/W(Y) of the playback light beam acrossthe track width (X) and along the track (Y) are shown in the figure. Asseen from the figure, the levels of the playback signal and thepush-pull signal do not depend largely on the shape of a pit except thecase where Wm=0.3 and Wi=0.2. There is no such pit depth as brings thelevels of the playback signal and the push-pull signal to the maximumlevel simultaneously. To minimize a decrease in the push-pull signallevel and obtain the maximum playback signal level, it is desirable fromFIG. 4 that the pit depth should be approximately λ/5, preferably in therange of λ/4.2 to λ/5.2. More specifically, the pit has a depth in arange of 1/4.2×λ/n to 1/5.2×λ/n, where n is a refractive index of thesubstrate.

FIG. 5 diagrammatically shows an MTF of the playback optical system andthe pit arrangement on the optical disk used to evaluate crosstalkbetween adjacent tracks. In the figure, the spots 50 and 51 of theplayback light beam represent a spot at the center (A) of a pit and aspot passing through position (B) a distance of td away from the centerof a pit. MTF is expressed by the power of the basic frequency componentof the output signal from the photosensor obtained when the beam spotpasses through the center of a pit. Crosstalk is expressed by the powerof the basic frequency component of the output signal from thephotosensor obtained when the beam spot passes through position B.

FIG. 6 shows the MTF and the crosstalk characteristics when the trackpitch Pt is fixed at 0.72 μm, the depth hm of the pit 10 is fixed at 0.2μm, and the upper width Wm and lower width Wi of the pit 10 are changedvariously, with the spatial frequency on the abscissa and MTF andcrosstalk on the ordinate. The values of beam packing factors A/W(X) andA/W(Y) of the playback light beam across the track width (X) and alongthe track (Y) are shown in the figure. In the figure, the MTF produces adifference of 1 to 2 dB depending on the pit shape, which is not toolarge. In contrast, it can be seen that crosstalk changes greatly withthe pit shape.

FIG. 7 shows the MTF and the crosstalk characteristics under the sameconditions as in FIG. 6 except that λ is set at 0.650 μm.

FIG. 8 shows the MTF and the crosstalk characteristics when the trackpitch Pt is fixed at 0.72 μm, the depth hm of the pit 10 is fixed at 0.2μm, and the upper width Wm and the angle θ of the inner wall 11 of thepit 10 are varied, with the spatial frequency on the abscissa and MTFand crosstalk on the ordinate. The values of beam packing factors A/W(X)and A/W(Y) of the playback light beam across the track width (X) andalong the track (Y) are shown in the figure.

FIG. 9 shows the MTF and the crosstalk characteristics under the sameconditions as in FIG. 8 except that the upper width Wm of the pit 10 isfixed at 0.35 μm and only the angle θ of the inner wall 11 of the pit 10is changed variously.

It is assumed that RLL (Run-Length Limited) scheme are used as amodulation scheme of information recorded on the optical disk. It isnecessary in this scheme to pay attention to crosstalk due to lowfrequency components when the longest pit is sensed. Although thecrosstalk characteristics shown in FIGS. 6 to 9 are for no tilt, tiltsactually have to be taken into account. FIG. 10 shows the MTF and thecrosstalk characteristics when tilts are taken into account. As can beseen from the figure, when tilts are taken into account, the MTF almostremains unchanged, but the amount of crosstalk increases, and theconditions for determining the parameters of a pit become more strict.

In the system design of an optical disk apparatus, if a tilt due to thewarp of the optical disk itself and a tilt due to the apparatus areconsidered to be 5 mrad and 3 mrad, respectively, a tilt of about 8 mradin total must be tolerated. According to the simulation in FIG. 10, theamount of crosstalk can be suppressed to values less than -20 dBrequired in practical use in the tilt range of ±10 mrad for the samespatial frequency. This shows that a wavelength of 0.685 μm and a trackpitch of 0.72 μm are reasonable.

The evaluation of FIGS. 6 to 9 shows that the amount of crosstalk (thedifference in MTF value between the MTF characteristic and the crosstalkcharacteristic) is 20 dB or less until Wm=0.45 μm in the case where thetrack pitch shown in FIGS. 6 and 8 is 0.72 μm. In contrast, when Wm=0.5μm, the amount of crosstalk at low frequencies increases rapidly,exceeding -20 dB. The MTF characteristics are relatively good untilWm=0.3 μm, but deteriorates sharply when Wm is less than 0.3 μm.Therefore, the range of Wm=0.3 μm to 0.45 μm is reasonable.

The results mentioned above show that when the pit shape across thetrack width is standardized with a wavelength of 0.685 μm and NA=0.6(i.e., λ/NA=1.14), it is desirable that the upper width of a pit shouldbe (0.3 to 0.45)×λ/NA/1.14 μm, and the lower width of the pit should be(0.2 to 0.25)×λ/NA/1.14 μm, or that the upper width Wm should be (0.3 to0.45)×λ/NA/1.14 μm and the angle θ of the inner wall should be in therange of 50° to 70°. Specifically, when the track pitch Pt is selectedin the range of (0.72 to 0.8)×λ/NA/1.14 μm and the track pitch is madesmaller than the beam spot diameter of the playback light beam,selecting the upper width Wm and lower width Wi of the pit or the upperwidth Wm of the pit and the angle θ of the inner wall in the aboveranges enables the amount of crosstalk to be suppressed to values lessthan -20 dB required in practical use in the tilt range of ±10 mradexpected in an actual optical disk apparatus, thereby achieving aremarkable improvement in the recording density. As a result, bycombining these track pitch and pit shape, the laser diode with awavelength of 0.685 μm, for example, as mentioned earlier, the 0.6 mmthick substrate, the object lens with NA=0.6, the subject of recordingtwo hours of compressed moving-picture information by MPEG with the CDsize can be achieved easily.

The parameters used in the explanation of the invention are obtainedthrough calculations on the assumption that the pit is in the form of anideal trapezoid. Actually, however, the pit does not take the form of anaccurate trapezoid, but curves at its corners as shown in FIG. 13.Therefore, the parameters for the ideally trapezoidal pit, or the modelpit, differ from those for the actual pit. FIG. 14 shows the differencebetween the bottom groove width Wi and the top groove width Wg for themodel pit and the actual pit. As seen from FIG. 14, the value of Wi forthe model pit ranges from 0.2 to 0.32 μm; the value of Wg for the modelpit ranges from 0.3 to 0.45 μm, whereas the value of wg for the actualpit varies from 0.3 to 0.5 μm. Furthermore, the angle of inclination θof the pit is as follows. As shown in FIG. 15, an angle of inclinationof the mode pit is in the range of 50° to 70°, whereas that of theactual pit in the range of 30° to 60°.

Hereinafter, the structure of an optical disk according to the presentinvention will be described. FIGS. 16A and 16B are a perspective viewand sectional view of a double-sided optical disk 100, respectively. Onesurface of each of transparent substrates 101 and 102 is embossed withpits made of light-transmitting resin such as polycarbonate or acrylicresin and is coated with a reflecting film 103 and a reflecting film 104(e.g., of aluminum), respectively. On these films, protective films 105and 106 are formed. The thickness of the transparent substrates 101 and102 is 0.6 mm. The transparent substrates 101 and 102, whose protectivefilms 105 and 106 are allowed to face each other, are laminated togetherwith an adhesion layer 107 with a thickness of several tens of μm madeof a thermoset adhesive. In the center of the optical disk 100, a hole108 is made for clamping. Around the hole, a clamping zone 109 isprovided. A playback light beam 110 is emitted from a laser diode (notshown), passes through the playback optical system, enters an opticaldisk 100 via an object lens 111 from the transparent substrates 101 and102, and is focused as a small beam spot on the reflecting films 103 and104.

FIG. 17 shows an example of an optical disk apparatus which reproducescompressed moving-picture information by using the above-mentionedoptical disk 100. Because the optical disk 100 uses the substrates 101and 102 which are as thin as 0.6 mm and consequently are less immune todust or dirt on their surfaces than a CD using a 1.2 mm thick substrate,the disk 100 is housed in a cartridge 200. By housing the optical disk100 in the cartridge 200, attention need not be paid to the way ofholding the disk, dust, fingerprints, etc. as with CDs, which is helpfulin handling and carrying. When the disk is exposed as is a CD, theability to correct errors must be determined, taking into account anunexpected accident such as a flaw. Use of the cartridge 200, however,makes such a consideration unnecessary. Therefore, it is possible to usethe LDC read Solomon error correction technique in sectors as used in arecordable optical disk. As a result of this, for example, when anoptical disk is formatted in units of 2 kbyte to 4 kbyte, the recordingefficiency can be increased by more than 10% as compared with the CD.

When the 4/9 modulation method is used as a modulation method for theinformation recorded on the optical disk 100, the track pitch on theoptical disk 100 is 0.72 μm, and the pit pitch is 0.96 μm, it isexpected that the pit density ratio is 3.84 times as high as theconventional CD format, the modulation efficiency is increased by 20%,and the format efficiency is increased by 10%. Consequently, thecapacity can be expected to increase by a total of 5.1 times. Asdescribed earlier, when moving-picture information such as a movie isreproduced with a picture quality as high as S-VHS, this requires a rateof 4.5 Mbps including sound, so that the capacity required for two hoursof reproduction is 4 Gbyte. Because of the aforementioned capacityincrease by 5.1 times, the 4-Gbyte capacity can be realized on one sideof the disk. Furthermore, as shown in FIGS. 16A and 16B, a singledouble-sided optical disk alone enables four hours of recording at amaximum.

In FIG. 17, the optical disk 100 is chucked by a tapered cone 220 androtated by a spindle motor 201. The spindle motor 201 is driven by aspindle motor driver circuit 202. The playback optical system isconstructed as follows.

An objective lens 203 is placed so as to face the optical disk 100. Theobjective lens 203 can be moved along the optical axis by a focus coil204 and across the track width by a tracking coil 205. The wavelength ofa laser diode 207 driven by a laser diode (LD) driver 206 is 0.685 μm.The light beam emitted from the laser diode 207 is made into parallelluminous flux by a collimate lens 208 and then enters a polarizationbeam splitter 209. The light beam emitted from the laser diode 207 hasgenerally an elliptic far field pattern. Therefore, when a round patternis needed, a beam shaping prism has only to be placed after thecollimate lens 208. The light beam passed through the polarization beamsplitter 209 is focused by the objective lens 203 onto the optical disk100.

The light reflected by the reflecting film on the optical disk 100passes back through the objective lens 203 in the opposite direction tothe incident light beam, is reflected by the polarization beam splitter209, and enters a photosensor 212 via the sensing optical systemcomposed of a condenser lens 210 and a cylindrical lens 211. Thephotosensor 212, for example is a 4-quadrant photosensor. The four senseoutputs of the photosensor are input to an amplifier array 213containing an amplifier and an adder-subtracter, which produces a focuserror signal, tracking error signal, and playback signal. The trackingerror signal is obtained by, for example, a push-pull technique in theform of a push-pull signal as described earlier. The focus error signaland tracking error signal are supplied to the focus coil 204 and thetracking coil 205 via a servo controller 214. As a result of this, theobjective lens 203 is moved along the optical axis and across the trackwidth, thereby focusing the light beam onto the surface of thereflecting film serving as the recording surface of the optical disk100, and tracking the target track.

The playback signal from the amplifier array 213 is input to a signalprocessing circuit 215, which binarizes the input and then senses datapulses. The sensed data pulses are inputted to a disk controller 216,which decodes the format, corrects errors, and then supplies theresulting signal as a bit stream of moving-picture information to anMPEG2 decoder/controller 217. Because the data obtained by compressing(encoding) the moving-picture information according to the MPEG2standards is recorded on the optical disk 100, the MPEG2decoder/controller 217 expands (decodes) the bit stream input toreproduce the original moving-picture information. The reproducedmoving-picture information is supplied to a video signal generatorcircuit 218, which adds a blanking signal etc. to produce a video signalin a specific television format. The techniques related to MPEG2 havebeen disclosed in U.S. Pat. No. 5,317,397 and U.S. patent applicationSer. No. 08/197,862.

As explained above, the optical disk according to the present inventionhas such an optimal pit shape (the upper and lower widths of a pit orthe upper width of a pit and the angle of the pit's inner wall) as makesit possible to set the track pitch to a smaller value than the spotdiameter of the playback light beam and decrease crosstalk betweenadjacent tracks to a level required for practical use. As a result, withthe optical disk, the track density can be made by about 1.5 times ashigh as the conventional CD and the sufficient levels of the playbacksignal and the push-pull signal used for tracking can be assured.

Accordingly, with the present invention, as shown in the embodimentsdescribed above, the capacity about five times that of the conventionalCD can be realized even using the normal CD size, for example. Inaddition, 4 Mbps of compressed moving-picture information with a picturequality as good as that of a high quality VTR, including sound, can bestored for two hours, which is very useful in practical use.

In the above-described embodiment, the values of a track pitch and upperand bottom widths in a pit are set as ones obtained by multiplying λ/NA(λ: wavelength (μm) and NA: numerical aperture of objective lens) byproportional coefficients within ranges respectively set incorrespondence with λ/NA and the numerical aperture. This makes itpossible to set a parameter suitable for recording information at highdensity without depending on a wavelength of light to be used or anumerical aperture of an objective lens.

However, a condition in the embodiment is set on the basis of thecondition that aberration generated by inclination of a disk isequivalent to a wavelength as a reference. This will be described indetail by referring to FIG. 18.

FIG. 18 shows a result of calculating aberration generated byinclination of a disk with respect to several wavelengths. An abscissaindicates disk tilt angles (mrad) in a radial direction while anordinate indicates aberration rms (root mean square) values bywavelength units. The aberration rms values are nearly proportional tothe disk tilt angles while inversely proportional to wavelengths. Inparts in which the tilt angle is smaller than 20 mrad, the aberrationrms value, that is, Wrms, is obtained by the following expression:

    Wrms=3.58×10.sup.-3 θ (mrad)/λ (μm)

For instance, when a wavelength is 0.65 μm and a disk tilt angle is 10mrad, Wrms=0.0551 λ.

The condition in the above-described embodiment is a value obtained onthe basis of the condition that an allowable tilt angle of 10 mrad isgiven when a wavelength is in the vicinity of 0.65 μm. This means, inother words, that a tilt angle of 10 mrad is allowed with respect to awavelength in the vicinity thereof, and in case where light of a shorterwavelength is used, an allowable tilt angle is made smaller. Thisrelationship is represented by the following expression:

    θ (mrad)=15.4 λ (μm)

To explain this by referring to FIG. 18, if there is disk inclination of10 mrad with a wavelength of 0.65 μm, an aberration rms value is 0.0551λ. As an example, in case where light generated by SHG (Second HarmonicGeneration) combining YVO₄ and KTP, a wavelength thereof is 0.532 μm.However, in order to limit aberration to the same rms value as in thecase of the above-described condition, that is, 0.0551 λ, at this time,a disk tilt angle to be allowed is 8.2 mrad. When a wavelength isshortened to 0.42 μm or 0.36 μm by using materials such as GaN and thelike, a tilt angle to be allowed is further reduced to 6.5 mrad or 5.5mrad.

If recording density is to be improved in such a short wavelength, anallowable disk tilt angle is accordingly made smaller. Thus, a requestfor improvement in machine accuracy including accurate formation of adisk, accuracy of a spindle motor and a turntable, chucking accuracy ofa disk, etc., will be stronger making it difficult to provide aninexpensive apparatus.

The preferred embodiment was devised in view of this situation and anoptical disk apparatus having recording density as high as possible isprovided without increasing a demand on machine accuracy. This opticaldisk apparatus achieves high recording density by keeping an allowabledisk tilt angle constant, e.g. 10 mrad. In this case, since the amountof aberration generated in each wavelength at 10 mrad increases, inorder to allow such an increase in aberration it is necessary to set alarger parameter used to set recording density for a track pitch and adetection window width. In FIG. 18, if wavelengths of light are setsmall, e.g. 0.532 μm, 0.42 μm and 0.36 μm, the amount of aberrationgenerated at the disk inclination of 10 mrad gradually increases to0.0673 λ, 0.0852 λ and 0.0994 λ. Allowing such large aberrationcorrespondingly causes, if expressed in terms of an optical system of0.65 μm, the allowable values of tilt angles to be such large ones as12.3 mrad, 15.5 mrad and 18.1 mrad. This conversion is represented bythe following expression based on the above-described proportionalrelationship between the amount of aberration and an allowable tiltangle:

    θ eq (mrad)=6.5/λ×(μm)

Here, θ eq indicates an angle at which aberration equivalent to thatgenerated when there is inclination of 10 mrad in each wavelength isgenerated in the optical system of a wavelength 0.65 μm. This angle θ eqincreases inversely proportional to the wavelength.

To allow such a large tilt angle, it is necessary to set the value ofparameters for determining recording density, e.g., a track pitch, adetection window width and so on, at a value larger than that setproportionally to (λ/NA).

FIG. 19 shows a relationship between disk tilt angles and windowoccupancy ratios. As typical values in the above-described embodiment, awavelength is set at 0.65 μm, a numerical aperture at 0.6, a track pitchat 0.7525 μm, a pit upper width at 0.35 μm, a pit bottom width at 0.2μm, a detection window width at 0.134 μm assuming that a modulated codescheme of d=2 (where d is the minimum run length of `0` for the RLLmodulation code) is adopted and a pit depth at 1/5 of a value obtainedby dividing a wavelength λ by a substrate refractive index n. Arelationship between disk tilt angles and window occupancy ratios inthis case is represented by a curve of (1.0 times). Curves of (1.1times) and (1.2 times) are obtained when a track pitch, a pit upperwidth, a pit bottom width and a detecting window width are expandedrespectively by 1.1 and 1.2 times. Though recording density declines to1/1.1² and 1/1.2², disk tilt angles to be allowed increase instead.

In FIG. 19, window occupancy ratios indicated by an ordinate are valuesobtained by calculating reproducing signals based on the scalardiffraction theory, including a lower limit achievable when cross talkfrom an adjacent track is to be considered in addition to inter symbolinterferences (ISI) by various pit patterns generated under therestrictions of modulated codes and in each case jitters are to bereduced by using an optimum equalizing circuit. It requires a great dealof time to calculate these window occupancy ratios and this is madepossible only with the development of a calculation program based onhigh-speed algorithm. Also, since NA is large, even in calculation ofaberration caused by inclination of a substrate, a more accurateevaluating method based on ray tracing is used rather than a usuallyused approximate expression.

FIG. 20 is a view outgrown from FIG. 19 and obtained by plottingmultiplication ratios necessary for allowable disk tilt angles with awindow occupancy ratio of 80% as a reference.

According to the above-described preferred embodiment, a procedure fordesigning an optical disk apparatus is as follows:

First, by referring to FIG. 18, a determination is to be made on whetherit is possible or not to set small an allowable value of a disk tiltangle so as to keep aberration within that obtained when a wavelength is0.65 μm and a disk tilt angle is 10 mrad. If it is possible, parametersfor a track pitch, a pit upper width, a pit bottom width and a detectionwindow width may be determined based on the above-described embodiment.On the other hand, if it is impossible to set an allowable disk tiltangle so small, the value (0.08) of the amount of aberration to beallowed is read from a wavelength to be used (e.g. 0.42 μm) and anallowable disk tilt angle (e.g. 10.0 mrad) in FIG. 18, and in thisamount of aberration (0.08) a disk tilt angle (15.0 mrad) correspondingto the amount of aberration obtained in an optical system in which awavelength is 0.65 μm is to be considered as an allowable disk tiltangle therein. Then, a magnification ratio (1.2) necessary to realizethis allowable disk tilt angle (15.0 mrad) is to be read from FIG. 20.Lastly, by multiplying each parameter value set according to theabove-described embodiment by this multiplication ratio (1.2), it ispossible to realize an optical disk capable of securing a practical disktilt angle to be allowed even in a short wavelength.

Furthermore, in the description of the embodiment, thickness of asubstrate was 0.6 mm. However, since aberration is proportional tothickness of a substrate, it is possible to set a parameter bycalculating proportion when thickness of a substrate is one other thanthe above. For instance, if a substrate has thickness of 0.4 mm, a valueof aberration is 2/3 times as that in FIG. 18 and thus aberration may beset according to this multiplication. In other words, as describedabove, since the amount of aberration is inversely proportional to awavelength, if this condition is used, multiplication of substratethickness by λ means that a wavelength is multiplied by 1/λ in terms ofan aberration amount. Therefore, by performing conversion under thiscondition, it is possible to determine multiplication ratios withrespect to parameters for a track pitch, a pit upper width and bottomwidth set according to the above-described embodiment as in the case ofthe procedure.

In case where thickness of a substrate is 600 μm and a wavelength is μm,as described above, a relationship, θ eq (mrad)=6.5/λ (μm), holds. Incase where thickness of a substrate is d_(s) μm, however, the followingrelationship holds: ##EQU1## According to this relationship, a measureof an abscissa is added to the upper part of a graph at d_(s) /λ in FIG.20. The graph in FIG. 20 is made based on calculation taking awavelength of 0.65 μm as an example. If the measure on the upper part isused, however, without performing conversion via parameters of awavelength 0.65 μm scheme, it is possible to read necessarymultiplication ratios by calculating proportion between a substrate anda wavelength which are actually used.

Further in detail, in the above-described embodiment, ranges of a trackpitch, upper and bottom pit widths and depth are set in the followingway:

Track pitch: (0.72 to 0.8)×(λ/NA)/1.14 μm

Upper width: (0.3 to 0.5)×(λ/NA)/1.14 μm

Bottom width: (0.2 to 0.32)×(λ/NA)/1.14 μm

Depth: (1/4.2×λ/n) to (1/5.2×λ/n)

These parameters are values when a wavelength is 0.65 μm, and if awavelength of used laser light is shorter than this, these parametersare multiplied by α. That is, coefficients are respectively multipliedby α as in the following expression:

Track pitch: (0.72 to 0.8)α×(λ/NA)/1.14 μm

Upper width: (0.3 to 0.5)α×(λ/NA)/1.14 μm

Bottom width:(0.2 to 0.32)α×(λ/NA)/1.14 μm

Depth: (1/4.2×λ/n) to (1/5.2 λ/n).

This multiple can be read from the graph in FIG. 20. However, anapproximate expression like the following may be applied to designing.That is, if an abscissa indicates allowable disk tilt angles θ eq (mrad)expressed in terms of 0.65 μm and an ordinate indicates dimensionalmultiplication ratios α, α is represented by the following expression:

    α=0.002236×θ eq.sup.2 -0.01575×θ eq+0.934

If by using substrate thickness d_(s) and a wavelength λ this expressionof α is replaced by the above-described expression, that is,

    θ eq (mrad)=1.083×10.sup.-2 ×(d.sub.s /λ)

α is a value obtained by the following expression:

    α=2.623×10.sup.-7 ×(d.sub.s /λ).sup.2 -1.706×10.sup.-4 (d.sub.s /λ)+0.934

According to this expression, an optimum multiplication ratio whensubstrate thickness and a wavelength are both changed can be obtained asa function therebetween.

The above embodiment sets the allowable tilt angle at 10 mrad. However,the present invention can be applied to another tilt angle. In thiscase, θ eq and α are represented by the following expressions.

    θ eq (mrad)=1.083×10.sup.-3 ×(θ.sub.A d.sub.s /λ)

where θ_(A) (mrad): allowable tilt angle.

    α=2.623×10.sup.-9 ×(θ.sub.A d.sub.s /λ).sup.2 -1.706×10.sup.-5 (θ.sub.A d.sub.s /λ)+0.934.

What is claimed is:
 1. An optical disk comprising:a circular substratehaving a predetermined diameter and a predetermined thickness, saidsubstrate comprising information recorded as a plurality of pit trainsformed on said circular substrate with a track pitch, each said pittrain comprising a plurality of pits; and a reflecting layer formed onsaid substrate; wherein said information is reproduced by said pittrains being irradiated with a light beam via an objective lens, saidobjective lens having a numerical aperture NA, and wherein: said trackpitch is in the range of (0.72 to 0.8)α×(λ/NA)/1.14 μm when a wavelengthof said light beam is λ μm; each of said pits is scaled by themultiplication ratio α, said multiplication ratio α being used to secureallowable tilt angles of said optical disk and being obtained by2.623×10⁻⁷ ×(d_(s) /λ)² -1.706×10⁻⁴ (d_(s) /λ)+0.934 m, where d_(s) issaid predetermined thickness in μm and is equal to 600 μm; a radial tiltof said optical disk is not more than 9.5 mrad; and said predetermineddiameter is 120 mm.
 2. The optical disk according to claim 1, furthercomprising:a secondary substrate comprising secondary informationrecorded as a plurality of second pit trains formed on said secondarysubstrate at said track pitch, each said second pit train comprising aplurality of second pits; and a secondary reflecting layer formed onsaid secondary substrate, said reflecting layer and said secondaryreflecting layer being disposed between said substrate and saidsecondary substrate.
 3. The optical disk according to claim 2, furthercomprising a pair of protective films formed on said reflecting layerand said secondary reflecting layer, respectively, and positionedbetween said reflecting layer and said secondary reflecting layer. 4.The optical disk according to claim 3, further comprising an adhesionlayer formed between said protective films to adhere said protectivefilms to each other.
 5. An optical disk apparatus comprising:an opticaldisk comprising a circular substrate and a reflecting layer formed onsaid substrate, said substrate having a predetermined diameter and apredetermined thickness and comprising information recorded as aplurality of pit trains formed on said substrate with a track pitch,each said pit train comprising a plurality of pits; an objective lensdisposed so as to face said optical disk, said objective lens having anumerical aperture NA; means for projecting a light beam onto saidoptical disk via said objective lens; and means for sensing reflectedlight of said light beam projected onto said optical disk by saidprojecting means to reproduce said information recorded on said opticaldisk; wherein: said track pitch is in the range of (0.72 to0.8)α×(λ/NA)/1.14 μm when a wavelength of said light beam is λ μm; eachof said pits is scaled by the multiplication ratio α, saidmultiplication ratio being used to secure allowable tilt angles of saidoptical disk and being obtained by 2.623×10⁻⁷ ×(d_(s) /λ)² -1.706×10⁻⁴(d_(s) /λ)+0.934 m, where d_(s) is said predetermined thickness in μmand is equal to 600 μm; a radial tilt of said optical disk is not morethan 9.5 mrad; and said predetermined diameter is 120 mm.
 6. The opticaldisk apparatus according to claim 5, wherein said sensing means producesa push-pull signal and a playback signal, said push-pull signalrepresenting a difference between signals sensed in at least two areasalong a track of said optical disk, and wherein each of said pits has adepth to enable both said push-pull signal and said playback signal tohave large levels.
 7. The optical disk apparatus according to claim 5,wherein said information, when recorded on said substrate, is compressedaccording to MPEG2, and wherein said sensing means has a function ofexpanding said information to reproduce said information.
 8. An opticaldisk comprising:a pair of transparent circular substrates with surfacesfacing one another, each of said transparent circular substrates havinginformation recorded as a plurality of pit trains formed on acorresponding one of said transparent circular substrates with a trackpitch, each said pit train comprising a plurality of pits; a pair ofreflecting layers coated on said facing surfaces of said circularsubstrates, respectively, a pair of protective layers formed on saidpair of reflecting layers, respectively; and an adhesive layer formedbetween said protective layers to adhere said protective layers to oneanother; wherein said information is reproduced by said pit trains beingirradiated with a light beam via an objective lens, said objective lenshaving a numerical aperture NA, and wherein: said track pitch is in therange of (0.72 to 0.8)α×(λ/NA)/1.14 μm when a wavelength of said lightbeam is λ μm; each of said pits is scaled by the multiplication ratio α,said multiplication ratio α being used to secure allowable tilt anglesof said optical disk and being obtained by 2.623×10⁻⁷ ×(d_(s) /λ)²-1.706×10⁻⁴ (d_(s) /λ)+0.934 m, where d_(s) is said predeterminedthickness in μm and is equal to 600 μm; a radial tilt of said opticaldisk is not more than 9.5 mrad; and said predetermined diameter is 120mm.